Unless you happen to be driving on a road where a DUI checkpoint has been set up, officers need to have a reasonable suspicion that a driver is impaired in order to pull them over for suspected DUI.
Once a driver has been stopped, officers can administer a Breathalyzer test and other field sobriety tests to more accurately determine whether they're intoxicated. However, if an officer didn't have an adequate cause to have reasonable suspicion, any evidence obtained after the stop could be tossed out -- along with the case.
Examples of things that can provoke reasonable suspicion include erratic and unsafe driving behaviors such as:
- Drifting between lanes
- Straddling the center line
- A near collision with a car or object
- Frequent braking
- Driving very slowly or stopping for no reason
After an accident, officers also have the right to administer sobriety tests if they believe one or more drivers is under the influence even if they didn't witness erratic driving or the crash itself.
Further, if an officer has another legitimate reason to pull over a driver, such as a malfunctioning light, they can proceed with sobriety testing if the driver gives them reason to suspect that they're impaired when they approach them and begin speaking with them.
Note that reasonable suspicion that a driver is DUI doesn't give officers the right to arrest them. For that, they need probable cause that a person has likely committed a crime. That's what a Breathalyzer and other field sobriety tests can provide.
If you're facing DUI charges and you believe that you were pulled over without cause for reasonable suspicion and/or arrested without probable cause, you should discuss that with your attorney. They can determine whether the actions leading up to your arrest were appropriate. If not, they can work to get the charges dismissed.